The walls of hot gas-conducting combustion chambers, for example of gas turbine plants, require a thermal shielding of their supporting structure against attack by hot gas. The thermal shielding can be implemented for example by means of a hot-gas lining, e.g. in the form of a ceramic heat shield, disposed in front of the actual combustion-chamber wall. A hot-gas lining of this type is usually constructed from a number of metal or ceramic heat-shield elements with which the combustion-chamber wall is lined over its surface area. Compared with metal materials, ceramic materials are ideally suited for the construction of a hot-gas lining due to their high temperature stability, corrosion resistance and low thermal conductivity. A ceramic heat shield is described for example in EP 0 558 540 B1.
Due to material-specific thermal expansion properties and to the temperature differences typically arising in the operating context—say, between the ambient temperature when the gas-turbine plant is at a standstill and the maximum temperature at full loading—the heat transferability, particularly of ceramic heat shields, as a result of temperature-dependent expansion must be ensured so that thermal stresses that will destroy a heat shield do not occur as a result of the prevention of temperature-dependent expansion. Expansion gaps are therefore present between the individual heat-shield elements so as to enable thermal expansion of the heat-shield elements. For safety reasons, the expansion gaps are designed such that they are never fully closed even when the hot gas is at maximum temperature. It must therefore be ensured that the hot gas does not pass via the expansion gap to the supporting wall structure of the combustion chamber. In order to seal the expansion gaps against the ingress of hot gas, these expansion gaps are frequently flushed with a flow of sealing air flowing in the direction of the interior of the combustion chamber. Air which simultaneously serves as cooling air for cooling the retaining brackets holding the heat-shield elements is generally used as sealing air, and this leads among other things to the occurrence of temperature gradients in the region of the edges of a heat-shield element. As a consequence of flushing the expansion gaps with sealing air, the peripheral sides adjacent to the gaps are cooled, as is the cold side of the heat-shield elements. On the hot side of the heat-shield element, on the other hand, a high thermal input takes place due to the hot gas. Inside a heat-shield element, a three-dimensional temperature distribution therefore arises, which is characterized by a decline in temperature from the hot side to the cold side and by a temperature decline from central points of the heat-shield element toward the edges. Therefore, in ceramic heat-shield elements in particular, even where adjacent heat-shield elements do not touch one another, stresses can occur on the hot side which can lead to cracking and can thus adversely affect the service life of the heat-shield elements.
The heat-shield elements in a gas-turbine combustion chamber are typically embodied in a laminar manner and arranged parallel to the supporting structure. A temperature gradient that runs perpendicular to the surface of the supporting structure leads only to comparatively low thermal stresses, provided that, in respect of the ceramic heat-shield element in the assembled state, they can be prevented in the direction of the interior of the combustion chamber without hindrance.
A temperature gradient running parallel to the supporting structure, like that which runs from the peripheral surfaces of the heat-shield element to the center of the heat-shield element brings with it rapidly increased thermal stresses as a result of the rigidity of plate-like geometries with regard to deformations parallel to their largest area of projection surfaces. These thermal stresses result in the cold edges of the peripheral surfaces being placed under tension as a consequence of their comparatively low thermal expansion by hotter central areas which are subjected to greater thermal expansion. This tension can, where the tensile strength of a material is exceeded, lead to the formation of cracks which emanate from the edges of the heat-shield element and run toward central areas of the heat-shield element.
The cracks reduce the supporting cross section of the heat-shield element. The longer the cracks, the smaller is the supporting residual cross section of the heat-shield element. The thermally induced cracks can lengthen as a result of mechanical strains arising during operation of the gas-turbine plant, which leads to a further reduction of the residual cross section and may make the replacement of the heat-shield element necessary. Such mechanical strains occur for example where there are oscillating accelerations of the combustion-chamber wall which can be caused by combustion vibrations, that is vibrations in the combustion waste gases.
In order to reduce the demand for sealing air—and thus thermally induced stresses in heat-shield elements—, EP 1 302 723 A1 proposed arranging flow barriers in the expansion gaps. This can also lead to a reduction of the temperature gradient in the region of the edges. The introduction of flow barriers is not, however, always readily possible and also increases the complexity of a heat shield.
Alternative procedures consist in using heat-shield elements made of metal. While metal heat-shield elements exhibit greater resistivity to temperature fluctuations and mechanical loadings than ceramic heat-shield elements, they necessitate, in gas-turbine combustion chambers for example, costly cooling of the heat shield since they have a higher thermal conductivity than ceramic heat-shield elements. Moreover, metal heat-shield elements are more susceptible to corrosion and, on account of their lower temperature stability, cannot be exposed to such high temperatures as ceramic heat-shield elements.